The present invention relates to the speed change control unit of an automatic transmission for automobiles. In general, in a transmission for automobiles based on electric and hydraulic controls, the car speed, the throttle opening, etc., are electrically detected, the optimum gear shift stage is determined depending upon the running conditions of the automobile, and predetermined solenoid valves are operated in accordance with the output, so as to selectively operate a plurality of hydraulic servo means composed of clutches or brakes, while an oil pressure to be applied to the hydraulic servo is electrically controlled depending upon the driving state of the automobile so as to adjust the engaging or releasing speed of the hydraulic servo, whereby a speed change operation is achieved while the shock occasioned by the switching of transmission gear ratios is being moderated.
The speed change operation needs to be smoothly effected in all the driving states of the automobile. Since, however, the driving states of the automobile change largely, it is very difficult to hold an ideal speed change operation in correspondence with such changes at all times.
It is of special importance that a temperature change varies the characteristics of the working oil of the hydraulic servo greatly. Therefore, such a temperature change has a great influence on the operating characteristics of the hydraulic servo and is one cause for making it difficult to always realize the smooth speed change operation.